The activity of streptodornase, a deoxyribonuclease active protein component of the medicament Varidase(R) has been shown to be dependant on the presence and concentration of certain cation species. Anion species appear to have little effect on streptodornase activity. In particular, the divalent cations magnesium and calcium are essential for optimum activity whilst sodium ions are inhibitory. The overall effect of ion concentration on streptodornase activity is a result of a complex interplay between the various ion species. These effects and conditions are described and a possible explanation for the observed interplay is offered.